1. Field of the Invention
This invention relates to a radio wave absorber which is capable of efficiently absorbing radio waves such as, for example, microwaves and radar waves. The invention also relates to a method for making such an absorber.
2. Description of the Prior Art
It is said that the present day is the era of the highly information-oriented society. There is a flood of various information mediums, which are now increasing steadily. No exception is involved in broadcasts and communications utilizing radio waves. More particularly, not only broadcasts and communications such as a conventional FM broadcast, an AM broadcast, a VHF television broadcast, a microwave multiplex radio, a radar communication from ships or fishing boats and the like, but also broadcasts and communications such as a satellite broadcast, an aeronautical radio, a personal radio, a data communication through a satellite and the like spread and increase. At present, substantially all radio waves in all frequency ranges are utilized but several undesirable phenomena take place, including a radio interference, various radio troubles and malfunctions. These phenomena now cause a serious social and environmetal problems. Accordingly, there is a strong demand for a so-called EMC (electro-magnetic compatibility).
To this end, a number of wave absorbing materials have been proposed. For instance, there are known (1) a wave absorber which is composed of a sintered product of a soft ferrite having a large magnetic loss (a real number of the imaginary part .mu." of magnetic permeability, .mu.=.mu.'+i.mu."), (2) a wave absorber sheet which has a soft ferrite powder filled in a rubber or resin, and (3) a wave absorber sheet which has semiconductive carbon with a large dielectric loss (a real number of the imaginary part .epsilon." of dielectric constant, .epsilon.=.epsilon.'+i.epsilon.") including an ohmic loss, filled in a rubber or resin. However, these absorbers have various problems with respect to their properties and applications.
The wave absorber (1) is disadvantageous in that its specific gravity is as heavy as about 5.0. Moreover, since the absorber is made of the sintered product, it is difficult to work such as for cutting, punching, drilling and the like. Although the wave absorber (2) is better in processing than the absorber (1) and has been heretofore applied to parts of bridges because of the advantage in ease of the processing, it has a relatively large specific gravity not less than 3.5 and is unsatisfactory with respect to the radiation absorption property. Accordingly, this absorber has never been used later. The wave radiation absorber (3) is relatively light in weight but a good wave radiation property cannot be obtained unless the dispersion of carbon and the thickness of the sheet can be strictly controlled in a uniform manner. This presents a problem such as a difficulty in controlling the manufacture and the quality of the product, so that a product of a stable quality has not been mass-produced yet and is only attempted to manufacture. In addition, with the wave absorbers (1) through (3), a good wave absorption characteristic is substantially attained only in a narrow range. Among them, the wave absorbers (2) and (3) have such a mechanism as to absorb a radio wave by utilizing a .lambda./4 resonance phenomenon while backing with a metal sheet, so that a radio wave of a given frequency is only absorbed. This places a certain limitation on the application of the absorbers. Further, a satisfactory wave absorption property cannot be obtained unless the thickness of the absorber is over 3 mm for the wave absorption (1) and over 1 mm for the absorbers (2) and (3), with an attendant problem that the wave absorbers become thick and heavy.